Performance of First-Trimester Screening between Gestational Weeks 7 and 13

Screening for fetal chromosome abnormalities in the first trimester includes analysis of the serological markers pregnancy-associated plasma protein A (PAPP-A) and free beta human choriogonadotropin (free beta hCG). The blood sample is traditionally taken around week 12 of gestation, but the performance of earlier blood sampling is not well documented.